The Intel P-state driver is currently undocumented. Add some
documentation based on the cover-letter sent with the original series.

Cc: Dirk Brandewie <dirk.brandewie@gmail.com>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NRamkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>